feat(dart-modern-features): add list/split pattern discovery regexes and case/Set guardrails

[kevmoo]

Adds list/segment and String.split() discovery regexes and two Pragmatic Balance guardrails to dart-modern-features:

• Discovery > Pattern Matching Candidates:
  • Adds regexes for .length + index checks, .skip(1)/.sublist(1) tail slicing, and .split(...) followed by length/index checks.
• Pragmatic Balance:
  • #### 3. Use switch Expressions or Inline if-case for Boolean Returns: Documents that <expr> case <pattern> is a caseClause valid only in if/for/while headers (not a standalone boolean expression), preventing Unexpected token 'case' compile errors.
  • #### 4. Use Set.contains in when Guards over Long || Pattern Chains: Shows how to combine list destructuring with Set.contains in a when guard (case [final first, ...final rest] when allowedTopLevel.contains(first)) to avoid +1 per || cognitive complexity inflation (analytica.dart).
• Cross-links dart-use-pattern-matching.